The latest Weekly Shonen Jump magazine came with a rather surprising announcement. In One Piece: Super Grand Battle! X, if you scan in your Mario, Link, Fox or Luigi Amiibo you'll be able to unlock secret Nintendo costumes for some of the One Piece crew.
These are the following characters who can wear the special costumes:
Luffy - Mario, Luigi
Zoro - Link
Sanji - Fox
It's unclear whether more costumes can be unlocked for different characters using other Amiibo as of yet.